FIG. 1 illustrates one type of prior art forward converter. A forward converter is a DC/DC converter that uses a transformer to increase or decrease the output voltage relative to the input voltage (depending on the transformer winding ratio) and provide isolation for the load. In a forward converter, unlike a flyback converter, energy is passed to the output of the forward converter by transformer action during the primary side switch conduction phase.
The maximum output voltage of the forward converter is constrained by the isolation transformer T1 turns ratio Ns/Np, where Ns is the secondary side winding and Np is the primary side winding. Vout equals the PWM duty cycle*Ns/Np*Vin.
Generally, in FIG. 1, a pulse width modulation (PWM) controller IC 12 senses Vout using any type of isolated feedback circuit 13, such as a photodiode-photodetector optical sensor or a transformer to achieve isolation. The PWM controller IC 12 compares the feedback signal to a reference and adjusts the duty cycle of the power MOSFET M1 to match the feedback signal to the reference. More specifically, the PWM controller IC 12 generates fixed frequency pulses, having the required duty cycle, for controlling the power MOSFET M1 (or other type of transistor) and also for controlling the secondary side MOSFETs MFG and MCG in order to keep Vout at a predetermined regulated voltage. MFG refers to a forward gate transistor (also referred to herein as a forward MOSFET), and MCG refers to a catch gate transistor (also referred to herein as a catch MOSFET). A catch transistor is also called a synchronous rectifier and substitutes for a diode. A synchronous rectifier is more efficient than a diode since there is a lower voltage drop, and an output voltage can be lower by using a synchronous rectifier.
When the PWM controller IC 12 issues a pulse via its primary side output pin OUT to turn on the MOSFET M1, it also issues a pulse via its secondary side output pin SOUT to control the secondary side MOSFETs MFG and MCG. When MOSFET M1 is on, MOSFET MFG is on and MOSFET MCG is off. When MOSFET M1 is off, MOSFET MFG is off and MOSFET MCG is on. The pulses are precisely timed to ensure MOSFET MCG is off when MOSFET M1 turns on to avoid wasting energy. The control signals from the PWM controller IC 12 to the secondary side need to be isolated from the secondary side via the transformer T2. A secondary side controller IC 14 receives the control signals at its SYNC input and controls the MOSFETs MFG and MCG synchronously with the switching of the MOSFET M1.
When the forward MOSFET MFG is on, a ramping-up current flows through the output inductor Lout, and the output capacitor Cout smooths the ripple to create a DC output Vout. When the MOSFETs M1 and MFG are off and the MOSFET MCG is on, the MOSFET MCG causes a ramping-down current to flow through the inductor Lout.
The current though the MOSFET MCG is monitored by the secondary side controller IC 14 by detecting the voltage across it (CSN and CSP). If the current is about to reverse (CSP approximately equals CSN), the controller IC 14 turns the MOSFET MCG off, so as not to waste power.
When MOSFET M1 is off, a reset circuit 16, controlled by the PWW controller IC 12, resets the primary winding of the transformer T1 to a starting state, such as by temporarily connecting a series capacitor between the primary winding and ground during MOSFET M1's off time.
FIG. 2 illustrates typical control signals received and generated by the ICs 12 and 14 to generate a regulated Vout by controlling the fixed frequency duty cycle of the MOSFET M1.
One drawback of the prior art converter of FIG. 1 is that the transformer T2 adds cost and size to the converter. It also adds complexity to the system. What is needed is a forward converter that can synchronously control the secondary side MOSFETs (or other types of transistors) without the use of a separate transformer like the transformer T2.